Adult onset acid maltase deficiency. Distribution and progression of clinical and pathological abnormality in a family.

The adult onset form of acid maltase deficiency (Type IIb) clinically affects only skeletal muscle. Proximal weakness is more severe in legs than arms and involvement of respiratory muscles is prominent in about a third of the cases. In three siblings with the disease, the severity of limb weakness was related to age and duration of disease. Respiratory muscle involvement was a feature in two of the patients. The pathological abnormality was more marked in quadriceps than deltoid, and was strikingly patchy in distribution within these muscles. In one case, vacuolar change was not evident by light microscopy in two quadriceps biopsies taken 2 years apart. There is thus a spectrum of clinical and pathological expression of the disorder even in a single sibship.